1. Field of the Invention
An integrated wheel for a motorcycle is well known. Such an integrated wheel is formed of a hub, a rim and spokes in an integral manner. In the integrated wheel, to improve the rigidity in a rotating direction while suppressing an increase in weight, each of the spokes is inclined toward a wheel circumferential direction from the hub outwardly in a wheel radial direction.
2. Description of the Related Art
FIG. 22 shows a conventional motorcycle having a wheel configuration as disclosed in JP-A-2009-184435, both of a front wheel 200 and a rear wheel 201 have an integrated wheel configuration, and both of spokes 200a and 201a are inclined in the wheel circumferential direction. Specifically, the spokes 200a of the front wheel 200 are inclined toward a rotating direction R1 from a front hub 200b outwardly in the wheel radial direction, whereas the spokes 201a of the rear wheel 201 are inclined toward an inverse rotating direction R2 from a rear hub 201b outwardly in the wheel radial direction.
FIG. 23 is an enlarged view of the rear wheel 201 in FIG. 22, and as described above, all of the rear spokes 201a are inclined toward the inverse rotating direction R2 with respect to a straight line M1 connecting a wheel rotational center O1 and a coupling portion 201d between each of the spokes 201a and the hub 201b. 
A load is applied to the front and rear wheels 200 and 201 in FIG. 22 during driving so as to displace the rim relative to the hub around the wheel rotational center. By inclining the spokes 200a and 201a, above the load, a bending load is generated, that is, a load in a direction orthogonal to the wheel radial direction or a load in a direction orthogonal to a spoke longitudinal direction, as well as a compressive load in the spoke longitudinal direction or a pulling load, to the spokes 200a and 201a. Thus, as compared to the case where all loads are received by the rigidity of each spoke in the bending direction, the loads can be received by the rigidity in the compression and pulling direction, thereby preventing deformation of the spokes and improving the rigidity in the wheel rotating direction.
In a motorcycle, to properly inform the rider of the grip state of the tire, it is desired to improve the rigidity of the wheels. As a means of improving the rigidity of the wheels, it has been proposed to increase the number of spokes, or increase the size in the wheel circumferential direction or the thickness in the axial direction of the spokes. In this case, an increase in the number of the spokes or the size of the spokes leads to an increase in the weight of the wheels and in a moment of inertia in the wheel rotating direction.
However, in a motorcycle, unlike a four-wheeled vehicle, the vehicle body needs to be inclined for circling (cornering). When the moment of inertia of the wheels increases as described above, it is difficult to incline the vehicle body, making improvement of the circling performance of the vehicle body difficult.
In a motorcycle for racing, rapid acceleration through an operation of an accelerator and rapid deceleration through an operation of a brake are often performed, and as a result, a ratio of the load applied from the road surface to the front wheel and the rear wheel frequently varies. At acceleration, the ratio of the load applied from the road surface to the rear wheel, that is, a driving load to the rear wheel becomes large. Conversely, at deceleration through the operation of the brake, the ratio of the load applied from the road surface to the front wheel, that is, a braking load to the front wheel becomes large. Moreover, the driving load applied from the road surface to the rear wheel at acceleration is applied as a large load to the rear wheel 201 in FIG. 22 toward the inverse rotating direction R2 (FIG. 22), whereas the braking load applied from the road surface to the front wheel at braking is applied as a large load toward the rotating direction R1.
However, as shown in FIG. 22, when the spokes 201a of the rear wheel 201 on the driving side are inclined toward the inverse rotating direction R2 from the hub outwardly in the wheel radial direction, improvement of the rigidity in the rotating direction due to inclination of the spokes 201a cannot be expected so much at acceleration, in particular, rapid acceleration. That is, the ratio of the bending load applied to the spokes becomes large, disturbing improvement of the rigidity.
An object of the present invention is to provide a wheel for a motorcycle capable of improving the rigidity of the wheels about the rotational center while suppressing the increase in the weight of the wheels and maintaining the circling performance of the vehicle body.
Further, another object is to improve the rigidity for the driving load while reducing the weight of the rear wheel by receiving the driving load applied to the rear wheel as a driving wheel as a compressive load in the longitudinal direction of the rear spokes.